Controlling or keying arrangement



April 4, 1939. K. L. KING CONTROLLING 0R KEYING' ARRANGEMENT Filed April 24, I957 INVENTOR K. L. KING mu ow Q W 3 8&5. 2 a 5%? q. a N 6? k fiwz x58 Q \m R I, a? Mk\ a... Q Q L: 7 3 3 f my 4 W W fi m W 3 3 M u making: wwflv m f it w it n. x

ATTORNEY Patented Apr. 4, 1939 o CONTROLLING OB KEYING ARRANGEMENT Kenneth L. King, Mountain Lakes, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 24, 1937, Serial No. 138,803 In Great Britain July 28, 1936 I arrangements for high power radio transmitters and more particularly to arrangements of the kind in which keying or the starting and stop-- ping of transmission is effected by controlling the grid bias voltage of one or more power amplifier stages.

In order to utilize power amplifiers to full capacity it is customary to operate the vacuum tubes with considerable secondary emission of electrons from the grids. Commonly the current leaving the grid due to secondary emission exceeds the current entering, so that the result is a reverse grid current. These conditions accompany the application of large exciting voltages which cause the grid to swing strongly positive during a portion of each cycle. Since there is generally very little resistance in the. input circuits of amplifiers of this type, the grid current does not vusually operate to modify the grid bias.

When it is desired to control the output of the amplifier it is customary to do so by changin the grid bias. When it is desirable that the change in the output be gradual rather than sudden a smoothing arrangement such as a resistance-capacity filter in the grid circuit becomes necessary. Because of the flow of grid current through the resistance of the filter the grid bias is affected by the grid current. When secondary emission is large, so that the grid current decreases or' reverses with increasing g d voltage, the flow of grid current in the filter resistances produces an unstable grid bias which tends to become indefinitely more positive until overloading or damaging'of the tube results.

In accordance with the invention the filter resistance is shunted with a rectifier, poled in the proper direction to freely pass the reverse grid current. In this way the grid bias is effectively removed from the influence of the reverse current and is controlled by the grid current only when the latter is positive. As a result, the grid cannot become materially more positive than the normal bias imposed by the grid biasing source. The circuit is then stabilized and the vacuum tube is not subject to overloading.

In the accompanying drawing, Fig. 1 shows the embodiment of the invention in a radio telephone transmitter and Fig. 2 is a graph used in" explaining the invention.

In the system of Fig. 1, a carrier source I is arranged to supply a carrier wave to a modulator 2. For effecting modulation of the carrier wave, a signal source 3 is connected to the modulator,

a delay network I being interposed to permit any desired switching operations to be completed while the signal wave traverses the network. The output circuit of the modulator is connected to a train of high frequency amplifying stages of which two are shown. These are denoted generally by 5 5 and 6, the latter of which is coupled to an antenna 1 and ground 8. The signal source is also connected to a rectifier 9 which is in turn connected to the respective operating windings Ill and II of a pair of electromagnetic relays. The 10 armature l2 associated with winding I0 is provided with a front contact connected through a resistor l3 to an adjustablepoint in a grid biasing device exemplified by a battery ll. The armature I2 is also provided with a back contact which is connected through a resistor 15 to the negative terminal of the battery IL The armature It associated with the relay winding II is provided. with a front contact connected through a resistor ii to an adjustable point in a battery I8 and with a back contact which is connected through a resistor l9 to the negative terminal of the battery IS. The armature I2 is conductively connected through a lead 20 and conventional grid supply arrangements to the grids of the am-. plifying stage 5. The armature I6 is similarly connected through a lead 2| to the grids of the amplifying stage 6. The positive terminalsof the batteries I4 and it are grounded, as are likewise the cathodes of the amplifying stages and 6. A pair of condensers 22 and 23 are connected respectively from the leads 20 and 2| to ground. Finally, a valve or rectifier 24 is connected between the relay armature l6 and the adjustable point in the battery l8. For reasons 5 which will be explained below, the valve 24 is so directed as to .pass current freely from the grid conductor 2| to ground through the adjustable point of the battery [8 and to substantially block current in the reverse direction.

In the operation of the .system of Fig. 1, the carrier wave is supplied to the modulator by source I along with signals from source 3. The modulated wave is amplified in the stages 5 and 6' and impressed upon the antenna system I, 8 for 5 transmission to a distant point in known man ner. The radiation of carrier waves is subject to voice or signal control by means of the rectifier 9, relays l0 and II and the grid batteries [4 and it in a known manner. The control is such that the carrier is radiated only during the actual signaling, with a consequent saving of power in intervals between signal elements, that is, between successive speech trains. The carrier suppression is efiected by causing a paralyzing or' resistor I1.

When the system is in the inactive condition,

the relay windings Ill and II are normally deenergized and the relay armatures l2 and I6 are I resting against their respective back contacts. The condenser 22 is charged to the full battery voltage, biasing the stage 5' beyond cut-off and thereby substantially preventing transmission of the carrier. The condenser 23 is likewise fully charged, thus causing stage 6 to be biased in similar manner to a condition wherein transmission of carrier through this stage is substantially prevented.

When a signal is sent out by the source 3 the rectifier 9 is actuated to supply current suitable to energize relay windings Ill and II, thereby causing armatures l2 and 16 to be moved to their respective front contacts. Armature I2, then connects the grids of stage 5 to a suitable operating potential in the battary l4 through the resistor I3. A smooth and gradual development of the operating potential on the grids is assured by the joint action of the resistor l3 and condenser 22, whereby the condenser adjusts its charge, by means of an accompanying current through the resistor, until a new equilibrium with the battery is established, in a manner that is well understood. Meanwhile armature l6 has connected the grids of stage 6 to a suitable operating potential from the battery I8 through the Smoothing is here effected by the joint action of the condenser 23 and resistor ll.

When the operating potentials have been established the stages -5 and 6 are ready to amplify the output wave from the modulator.

When the signal current fails at the end of a signal element the relay windings l and H are immediately deenergized'. The armatureslZ and fall back, and the condensers 22 and 23 gradually charge up to the full battery voltages, with accompanying current through the resistors l5 and'l9, respectively. When this action has been completed, the stages 5 and 6 are blocked and the control system is in the normal-state, ready for the next signal element.

Fig. 2 shows in a general way the course of 'the grid current characteristic in stage 6 when various voltages of grid excitation are employed. In the region between the origin 0 and the point A the grid excitation is so low that the grids remain negative continually throughout the cycle of the high frequency wave. There is practically no grid current under this condition. With greater excitation grid current commences to flow and increases continuously between A and .B.

In the neighborhood of B the bombardment of the grid by the electrons from the filament is sufiiciently intense to produce noticeable sec ondary emission from the grid. As the secondary emission increases the grid current increases less, rapidly and eventually decreases as shown in the region BC. At C the grid current reverses, due to more electronsleaving the grid than entering. Further increase of the excitation increases the amount of reverse grid current until a certain maximum current is reached. Thereafter the current decreases and eventually a second reversal occurs.

In the operation of stage 6 as a power amplinal to the vacuum tubes contains a substantial amount of resistance, the fall of potential produced in this resistance by the reverse grid current will oppose the normal negative bias of the grid and, if large enough, may reduce the effective bias to zero or make it positive. The reduction of the grid bias tends to increase the reverse grid current with the result that a cumulative action occurs which produces a condition of instability. Parasitic oscillations may occur or the plate current of the vacuum tubes may rise to a destructive value.

In the amplifier illustrated, resistance ll, which controls the rate of growth of the amplideveloped to substantially its full value, the resistance capacity filter I1, 23, is fully efiective to control the growth of the amplifier output and to prevent the development of keying transients during the growth period. When the output has been fully developed and the secondary emission currents established, the rectifier 24 acts as a low resistance shunt to resistance l1 and thereby prevents an unstable condition being established. v

To further promote stability and also to avoid distortion of signals in the amplifying stage 6, it is desirable that the rectifier 24 should maintain the potential drop. across its terminals virtually constant regardless of current fluctuations over a wide range. For this reason a gas-filled diode tube'or mercury vapor rectifier is particularly suitable, although otherv forms. of valves andrectifiers may be used.

'I'hevalve 24. performs an additional function at the time when the signal ceases and the relay winding ll deenergizes. The armature l6 then breaks away from its main contact, but due to 'fied waves in stage 6, lies in the path of the the fact that the contact is paralleled by the valve 24 and because of the low resistance of the valve relative to the resistor lLthe main current is carried by the valve and the current which is broken at the contact is relatively small. Sparking at the contact is in this way substantially prevented. The deenergization of the windings l0 and H results in .agradual blocking of stages 5 and ii. The grid current through the valve 24 is thus safely reduced without imposing. severe requirements uponthe relay contact.

While the invention has been described in connection wtih its embodiment in a radio telephone transmitter it will be appreciated that it is equal- 1y applicable for telegraph purposes in systems where keying is effected by changing the grid bias of a power amplifier. Thus, in the circuit of Fig. 1, the signal source 3 may be a source of telegraph impulses in which case relays i0 and I I operate as keying relays for interrupting the carrier in'accordance with-the telegraph pulses.

nection thereto from the source 3 become unnecessary and may be omitted. Rectifier 9 may also be omitted since the relaysmay operate directly from the telegraph pulses.

What is claimed is:

1. In a two-stage amplifier, in each stage, a

condenser connected between the respective grid and cathode, a biasing source, a relay for changing the voltage of the respectivebiasing source; means for connecting the biasing source across 'the respective condenser, said means including series resistors for controlling the rate of charge and discharge of the condensers and a rectifier connected between the grid and the cathode of the second stage with its conductive direction from the amplifier grid toward the amplifier cathode.

2. In a two-stage amplifier, in .each stage, a

condenser connected betwen the respective grid and cathode, a biasing source, a relay for changing the voltage of the respective biasing source, means for connecting the biasingsource across the respective condenser, said means including series resistors for controlling the rate of charge and discharge of the condensers, and a unidirectionally conductive gas-filled tube connected between the grid and the cathode of the second stage with its conductive direction irom the amplifier grid toward the amplifier cathode.

3. In a two-stage amplifier, grid biasing means tapped for a plurality of voltages, individual relays for each stage to transfer the respective grids from one voltage tap of said biasing means to another, means for actuating said relays substantially simultaneously, a resistor in series with one of the voltage taps for the second stage amplifier, and a rectifier connected betwen the grid and cathode of said second stage amplifier and poled to conduct from the grid towards the cathode.

4. In a multistage amplifier, grid biasing means, relays in two of the stages for simultaneously changing the voltages applied to the respective grids by the biasing means, and a rectifier connected in parallel wtihthe contacts of the relay in the stage nearer the output, the congrid toward the cathode. 

